I would advise against marking mails without "from" as spam.
Internet RFC requires that DSN (Delivery Status Notice) not have a "from". So, if you eliminate such, you are dropping valid/useful mails.
AFAIK, That's not true.
Are you sure it says that, and you're not confusing the NULL return path requirements with message From: header requirements?
RFCs do require that DSN's have a NULL return path designated as <>.
However, this is neither a From header, nor is it a missing header.
